This invention is directed to a method of transmitting a signal using digital compression techniques. In the transmission of an audio signal, for example, radio broadcast transmission, cable transmission, satellite transmission and with recording devices the analog signal is converted into a digital signal with a certain resolution, transmitted in digital form and reconverted into an analog signal upon reception. A greater signal-to-noise ratio is achieved, in particular upon reproduction, by using digital transmission.
The band width required for the transmission of such a signal is essentially determined by the number of [scanning values] samples per time unit which are to be transmitted. The resolution is also a function of the number of [scanning values] samples transmitted.
In practice it is preferable to keep the transmission band width as narrow as possible in order to be able to transmit as many audio signals as possible simultaneously via a wide band channel. It would appear that the necessary band width can be reduced by decreasing the number of [scanning values] samples or the number of bits per [scanning value] sample. However, in general this measure results in a deterioration in the quality of the reproduction.
A method described in DE-OS 35 06 912, improves the quality of the reproduction by separating the digital audio signal into successive temporal segments and transforming the audio into a short-time spectrum which represents the spectral components of the signal for the respective time segments. Generally, in the short-time spectrum, for reasons of psychoacoustic laws, components which are not perceived by the listener, i.e., are irrelevant from a communications technology viewpoint, can be discovered more readily than in the time domain. Upon transmission these components are given less weight or are left out entirely. In doing this a considerable part of the otherwise necessary data can be left out so that the average bit rate can be considerably reduced.
To form the time segments, the signal is first evaluated in the temporal region (time domain) using an analysis window and after transformation, coding, transmission, decoding and inverse transformation, is finally evaluated using a synthesis window. The design of the analysis window influences the frequency resolution. The advantage of a high frequency resolution is that with narrow band signal components only a small amount of data is required for their coding, thereby achieving a very effective bit allocation, and the average data quantity which is needed for transmission is considerably reduced. Therefore, for windows with "hard" edges, such as exhibited by a rectangle, the frequency resolution is poor. This is because the spectral components caused by the extreme rise and fall of the signal at the start and end of the window are added to the spectrum of the original signal in the evaluated segment. However, the temporal segments can be joined to each other without overlaps.
With the method described in DE-OS 35 06 912, a window function with "softer" edges was already selected. Here, the start and the end of the analysis window follow a cosine [square] function and the corresponding regions of the synthesis window a sine [square] function. The central area of both windows has a constant value. The use of such a window function design results in an improved frequency resolution. However, in the region of the "soft" edges overlapping of the successive temporal segments is necessary, and this leads to an increase in the average bit rate due to the doubled transmission of the signals contained in this region.
A further improvement in the frequency resolution could be achieved by using an even lower edge gradient for the window function of the analysis window as well as by expanding the edge region within the window. However, with these measures increased overlapping with neighboring temporal segments is required.
If the edge region is expanded so far that the window functions no longer have a constant value in any region, then adjacent temporal segments must overlap each other by 50 per cent. This means that the number of [scanning values] samples and, accordingly the quantity of data, is doubled.
From the publication of J. P. Pfineen and A. B. Bradley "Analysis/Synthesis Filter Bank Design Based on Time Domain Aliasing Cancellation", IEEE Transactions, ASSP-34, No. 5, October 1986, pp. 1153 through 1161, and that of J. P. Princen, A. W. Johnson and A. B. Bradley "Suband/Transform Coding Using Filter Bank Design Based on Time Domain Aliasing Cancellation", IEEE Int. Conference on Acoustics, Speech and Signal Processing 1987, pp. 2161 through 2164, it is known with a 50 per cent overlap of successive temporal segments to reduce the quantity of data to the original value again, in that only every second [scanning value] sample is encoded. In the spatial domain every sample is encoded (if data reduction is not considered). Sub-sampling is performed in the spectral domain. The sub-sampling process is explained at page 1154 and 1155 of the Princen and Bradley reference noted above. This proposal is based on equal window functions for the analysis and synthesis windows. In the case of equal window functions, the aliasing components which appear upon [sub-scanning (]sub-sampling[)] can be compensated for by the synthesis window after the evaluation.
It was discovered that the frequency resolution can be raised by selecting larger overlapping regions if, at the same time, the signal is assessed with suitable analysis and synthesis windows. In order to reduce the climbing data rate, caused by the higher number of [scanning values] samples, to the original value, the sub-[scanning] would have to be performed using an even higher factor, whereby however, further aliasing components ensue.